As far as the barrel flats being blue that is not always correct. Purdey leaves theirs blued and I have confirmed that with them. A few years ago at an event a fellow from Holland and Holland looked at my Purdey and commented that it had been re-blued. That was a sign of his experience.
Now you can look for any engraving and see how light it may be from the buffing before re-blueing. But a good gunsmith will take careful precautions to prevent this.

Mike Proctor